In process automation, field transmitters are widely used which regulate or control process cycles in production plants.
Examples of field transmitters are level gages, which determine the level in a liquid container, pressure gages, which determine the pressure of a liquid or of a gas, e.g. in a pipeline, mass flow meters, which determine the mass flow of a liquid in a pipeline section, or valves, which regulate the flow in a pipeline section.
Field transmitters can essentially be divided into two groups, namely sensors, which determine a process variable such as level, pressure, mass flow or temperature, or actuators, which influence a process variable such as the flow in a pipeline section. One example of actuators are valves.
Generally, field transmitters are connected to a process control system by means of a data link, said process control system controlling the entire process cycle in a production plant.
Data are transmitted on this data line on the basis of the known standards, such as Hart, Profibus or Fieldbus.
The data link is used to forward the measured values supplied by a sensor to the process control system, where they are processed. The control commands from the process control system are likewise forwarded to the appropriate actuators via the data link, and said actuators then react accordingly and open or close a valve, for example.
The field transmitters are sold to the end customer, who uses them on the desired process component.
In some cases, the process components do not occupy the same space in a production plant, but instead are distributed, which means that it would be too complex to connect the field transmitters directly to a process control system, e.g. using a data bus system. Examples of such process components are distributed tanks for propellant or fuel, and lime containers for combating forest damage by acid rain, which are set up with a broad scatter in forest areas.
In this case, the data transmission to the process control system takes place by radio.
The end customer is actually interested only in the measured value supplied by the sensor. In his process control system, he requires only this value in order to be able to control the production plant.
The way in which the measured value is obtained is of no importance to the end customer. The crucial factor for the end customer is that he is provided with a reliable measured value.
Today, the end customer pays for the sensor and not for that which he actually requires, the measured value.
A faulty sensor supplies no measured values and is therefore of no use to the end customer. The end customer does not want to pay for a sensor which supplies no measured values.
Many end customers require the measured value at relatively short intervals of time; others require the measured value only relatively infrequently. However, both end customers pay the same amount for the sensor, even though they use it with different frequency.